2009 Volume 58 Issue 7 Pages 274-279
In order to understand the detailed corrosion process of carbon steel depending on pH in the neutral solution, corrosion film formation reaction on the steel surface in 0.01M NaHCO3 solution has been analyzed as a function of pH, the value of which was adjusted by bubbling a mixture of air and CO2 gas at an atmospheric pressure. It was found that precipitation ratio defined as the amount of precipitated film to the total corrosion loss increases with increase of pH, resulting in decrease of the total corrosion loss with increasing pH in the range between pH=3.5 and 10. Small addition of alloying elements such as Cu and Ni was also found to change the precipitation ratio. The film formation reaction is rationally explained by assuming that precipitation is decided by the solubility product of Fe(OH)2, Fe(OH)3 or FeCO3. The rate of corrosion was found to obey the parabolic low, suggesting that the diffusion of oxygen dissolved in the solution through the film controls the whole process.
